Recently, not only apparatus performance such as image quality or low noise of an image display device but also service lives of the apparatus and a component have received attention from the viewpoint of effectively utilizing the environment and resources. Regarding a projection type image display device typified by a projector, a light source such as a discharge lamp, and an optical component such as a liquid crystal panel and a deflecting plate are consumables, and how to increase the service hours has become one of the demands.
The increase in service life of the light source such as a discharge lamp is disclosed in Japanese Patent Unexamined Publication No. 2005-209572, for example. This document describes a discharge lighting device that is formed of a power control unit for controlling power of the light source and a lighting-time managing unit for measuring continuous driving time of the light source. This device can reduce effective value of the power by varying variation range and variation cycle of the power supplied to the light source in a variation waveform. Therefore, electrode abrasion causing discharge in normal use can be suppressed, and degradation of a glass unit, which is a component of the light source, due to continuous use at a high temperature can be also reduced. As a result, the service life of the light source can be increased.
In one of the methods of improving contrast in the image display device, a restrictor for controlling quantity of light is used. This method improves contrast feeling by further brightening bright video by opening the diaphragm and by further darkening dark video by closing the diaphragm. This technology is disclosed in Japanese Patent Unexamined Publication No. 2001-100699.
However, the conventional configuration has the following problem. When a driving method of achieving the service life extension of the light source is employed, the driving power is varied over time, the brightness of the light source fluctuates in response to the driving power, and hence the brightness on the projection screen fluctuates. As a result, even when the brightness of the video is uniform, the brightness on the projection screen is not stable.
A method such as light quantity control of controlling the diaphragm in interlock with a video signal or diaphragm control of a lens can improve the contrast feeling, but disadvantageously cannot eliminate influence of the brightness fluctuation from the light source.